1. Field of the Invention
The invention relates to springs requiring a high ratio of stored energy to moving mass, so that the spring can move a payload through a specified distance in a very short time. The invention is applicable in the field of automotive valve springs, and especially to high performance springs used to restore electric valve actuation solenoids to a central position between two holding electromagnets.
2. Description of the Prior Art
Springs used to control fast motions with high accelerations must be able to exert a force through a distance, i.e. to transfer energy, while contributing minimally to the moving mass of the system. While a high performance spring will accelerate a payload mass through a specified stroke distance (e.g., the stroke of an electric valve actuator) in some specified short time period (e.g., 3 milliseconds), a poorly designed spring cannot even move its own mass through the specified distance in the specified time, with no payload at all. In a spring-and-payload system, some fraction of the effective moving mass ends up being spring inertia, with the remaining mass being the true payload. It is generally an advantage to maximize the payload fraction of the total moving mass, but in valve applications for internal combustion engines, and especially in the design of electric valve actuators for internal combustion engines, a high payload mass fraction is especially critical to overall performance. When the payload mass fraction is low, the spring mass and total mass of the system necessarily go up, in order to make the spring big enough to accelerate and move the valve payload through a specified stroke in a specified time. With an inefficient spring (i.e. a spring with a low ratio of exchanged elastic energy to effective moving mass), an increase in actuation force must accompany the increase in moving spring mass, implying more mechanical work performed per stroke, with a larger mechanism and at increased energy losses. In engines with a mechanical valve drive train, the actuation mechanism is a rotating cam, whose mass is not part of the mass to be accelerated with the valve and spring. In electric valve actuators, by contrast, the actuation mechanism includes an armature whose mass adds to the payload to be accelerated and decelerated quickly, by spring action, in transit between holding positions at full-open and full-closed. When moving spring mass is added, spring force must to be added to keep acceleration at a specified level. The increase in spring force calls for an increase in electromagnetic holding force, which in turn calls for an increase in armature mass. One sees that excess moving spring mass propels an upward spiral of mass addition to satisfy engineering requirements. Conversely, a reduction in moving spring mass for the same valve mass, stroke, and transit time, propels a spiral of mass reductions until the designer is faced with a desirable pair of design alternatives: either to make a faster valve, or to keep valve transit speed the same while transferring spring mass savings over into additional armature magnetic material, permitting achievement of increased electromagnetic efficiency.
Practical considerations for many high performance springs, particularly for electric valve actuation, usually include operation without fatigue and with minimal mechanical wear, and also compactness of the spring. Surface wear in highly stressed regions of a spring must be avoided, since wear accelerates stress-related failure. Attachment to a wire spring is prone to create localized stress concentrations, especially if the spring is attached where it undergoes significant bending or torsional moments. By far the most common solution to these multiple design challenges has been the helical compression spring. The wire in a helical spring experiences mostly torsion when the spring is compressed (or stretched, in the case of a tension spring design). It is well known that spring wire will store more energy per unit mass in a torsional mode than in a bending mode, lending an advantage to the helical compression or tension spring approach. In high performance compression spring designs, the end of the spring generally flattens out into a holding cup with a rolling motion causing no rubbing. A smooth transition is achieved from working spring wire to supported spring wire, minimizing stress concentrations. Compression springs whose ends are ground flat achieve a very small mass fraction that is non-working end mass. While it is an effective design, the traditional helical compression spring with flat ends leaves room for improvement, especially when used for electric valve actuation. In the electric actuation context, the valve is not preloaded to a mechanical stop, but instead sits at a neutral position roughly midway between its travel limits, until magnetic forces move the valve away from that neutral position either to a full-open or full-close position. The overall spring configuration must therefore exert force in two directions, toward the neutral position from either side. This bi-directional force is achieved, in the present art, by pre-loading a pair (or more than one pair) of compression springs against one another, so that one spring does most of the pushing from one side of center, while the other spring does most of the pushing from the opposite side of center. One finds that each compression spring stores two components of elastic energy: a variable energy component that contributes to the bi-directional centering restoration force, and a fixed energy component that provides compression preload but does not contribute to the bi-directional restoration. This mechanical fixturing preload serves to keep the ends of the spring seated firmly in their cups, since the end attachment is designed to push but not pull. By contrast, a built in xe2x80x9cpreloadxe2x80x9d of compressive surface stress, as achieved by shot peening of a finished wire spring, can help the surface resist crack propagation, thus extending fatigue life. Mechanical fixturing preload, as used to stabilize spring material with a net unidirectional force toward a confining surface, is a disadvantage if it creates a stress bias in material that is also highly cyclically stressed by spring operation. The functional price paid for the preload is that the spring wire must store substantially more total energy in relation to the xe2x80x9cworkingxe2x80x9d energy that cycles in and out of the metal with each stroke. While metal fatigue is associated most strongly with the cyclic component of stored elastic energy, the static preload energy component takes its toll on the design, cutting significantly into the capacity for cyclic energy storage. Hence, one might inquire whether springs without static preload, operating over a range including tension and compression, might offer improved performance over paired preloaded compression springs. The invention to be described below embodies an affirmative response to this query.
It is not easy to design an end attachment for bi-directional push-pull operation, especially if the spring wire must be gripped at the radius of the helix, where high torsion forces tend to twist the wire in its attachment and cause wear and fatigue. The desirable action whereby a compression spring flattens smoothly into an end cup is lost when one attempts to design for both tension and compression. In tension spring designs, an approach to reducing wear and stress at the end attachment point is to bend or spiral the end of the wire inward toward the center-axis, thus reducing or eliminating the force-times-radius couple that puts the spring wire in torsion. Wire spiraled inward to a center-axis attachment need only be gripped for force transfer, avoiding the formidable problem of wear-free gripping of a wire subjected to variable torsional stress. Existing tension spring designs achieve this objective, but adaptations of this kind of approach for combining tension and compression are lacking.
An example of a helical spring used in both tension and compression is found in U.S. Pat. No. 5,117,869 by Kolchinsky. This patent described a double-acting push-pull solenoid used in conjunction with a spool type cartridge valve. A single helical spring restores the solenoid armature and the valve spool to their center positions from either side. Details of the spring attachment are not shown or discussed in this patent, though the spring is illustrated. There is no indication that Kolchinsky""s design achieves a high ratio of spring elastic energy to effective moving spring mass, an important issue to be addressed below. The armature shown in this patent is, in fact, quite massive in relation to its energy stroke, as compared with high performance solenoids used in automotive electric valve actuators. Thus, there is little indication of attention paid to moving mass reduction in this patent.
Another factor in electric valve actuator design is space occupied by the spring. The envelope of space available to the actuator is typically confined in the lateral dimension parallel to the engine crankshaft by the spacing between adjacent cylinders. Space taken up by a spring lying between the valve end and the actuator is space that must be spanned by the valve stem. As the valve stem is lengthened, the valve payload grows. An ideal valve actuation spring should reside entirely on the far side of the armature from the valve end, allowing the electromagnetic mechanism to be brought down as close as possible to the valve end. It would be advantageous to have such a xe2x80x9ctop-sidexe2x80x9d valve spring fit into a compact, low-profile package with limited lateral extension in the direction of the crankshaft, where cylinder spacing is a limiting factor. An ideal would be a shallow rectangular package with spring force derived from the middle of the package, while the spring is fixed at two or more points off-center. The spring should provide linear axial force without causing side forces or torsional moments.
It is an object of the present invention to provide a spring capable of a reversing push-pull force. It is a further object to minimize effective moving spring mass in relation to the push-pull work provided by the spring. A still further object is to achieve the above two objects in the context of electric valve actuation. In order to fit into a narrow and low-profile envelope, as is advantageous to the electric valve context, it is an object to provide a low profile spring fitting into a rectangular envelope and with the moving spring attachment at the center of this envelope, while fixed attachments are provided at two or more points off-center. A further related object is to provide a twin-spiral spring with two end regions of static attachment and a single center region of moving attachment, where torsion and bending forces from the two halves of the spring are in balance. A related object is to take advantage of the internal torsion and bending force balance in the wire by gripping the spring with a low-mass attachment subject only to delivered, reversing, linear spring force, with no torsional or bending force transfer being required of the attachment. Relating to such center-gripping for linear force transfer, it is an object to grip the spring wire with a soft material, e.g., rubber, compressed around the wire and complying to the bending and twisting motions of the wire without rubbing or generating localized stress concentrations. In order to avoid subjecting such soft gripping materials to damaging levels of negative-pressure and shear stresses, it is an object to compress and confine these soft materials in a thin annulus around the spring wire, such that reversing force transfer arises primarily from variable positive hydrostatic pressure developed in the soft material on alternating sides of the wire. It is an object to achieve properties described above with twin parallel side-by-side helices having the same handedness of rotation sense and joined by a crossover region gripped by a moving attachment. In an alternative embodiment, it is an object to achieve properties described above with twin in-line end-to-end parallel helices having the opposite handedness of rotation sense and joined by a crossover region gripped by a moving attachment.
These and other objects of the invention will be made clear by the description and drawings to follow.
A spring optimized especially for electric valve actuation consists of parallel side-by-side twin helices, formed from a single unbroken length of wire and joining where the wire at the end of one helix ceases to curve, crosses to the opposite helix, and then commences to curve in the opposite rotation sense, as viewed in plan, to make the second helix. When the plan-view rotation sense reverses, the direction of axial travel also reverses, such that if the xe2x80x9cbeginningxe2x80x9d spring helix has a particular handedness (e.g., right handed), then the xe2x80x9cendingxe2x80x9d spring helix will have the same handedness (e.g., also right handed). The halves of the spring on either side of the center attachment are typically of the same shape, the one half being the image of the other half rotated 180 degrees, without mirroring, about a center axis generally parallel to the axes of the twin helices. Two generally static attachments to the spring are made at the two ends of the wire, those ends lying in a common plane perpendicular to the axis of spring motion. Moving attachment to the spring is generally made to the middle of the wire, in the center of an xe2x80x9cSxe2x80x9d-shaped crossover from the one helix to the other, at a point midway between the axes of the two helical sections of the spring. Motion of this moving attachment is generally parallel to the center axis described above, with spring force acting along this axis, which is designated the axis of spring action. This moving attachment typically consists of a cylindrical clamp encircling the wire and squeezing the circumference of the wire center via a compressed flexible or elastomeric sleeve between the inside of the clamp and the outer surface of the wire. This flexible sleeve transfers reversing push-pull force between the spring and an axially reciprocating shaft, while torsion and bending forces in the gripped portion of spring wire are internally balanced and require no force transfer through the clamp. The flexible or elastomeric material of the sleeve interface allows the wire to twist and bend slightly within the sleeve without rubbing or opening gaps, as the soft material follows the strains in the wire surface. In a design to reduce bending and torsion forces in either region of attachment at a static spring end, the spring helix at an end preferably spirals smoothly inward from its maximum helical radius to an attachment near to or on the center-axis of the helix. In a preferred static end termination topology, the wire spirals inward to a fairly sharp bend at a small radius off-center, then straightens abruptly as it passes across the center-axis of the helix. A cylindrical clamp and elastic sleeve grip this straight end portion of the wire in similar manner to the center attachment just described. In an alternative embodiment, each wire end terminates in a small-radius loop near the helix center, this end loop being gripped over an arc length between two washers or formed cups clamped tightly together. In a second alternative static termination topology, each wire end terminates in a central axial segment that experiences little bending or torsion stress and may be rigidly gripped from the sides, or pinned through the middle, or threaded like a bolt end, for axial force transfer.
In an alternative embodiment, two helices formed from a single unbroken length of spring wire are wound to form a monotonic axial progression about a common axis from one end to the other. The resulting spring is gripped at the two ends and at the middle. The end attachments are generally fixed, while the middle attachment is generally the moving or xe2x80x9cactivexe2x80x9d attachment. The helices forming the two ends of the spring are of opposite handedness, one being right-handed and the other left-handed, while the clamped middle region joining these two helices is xe2x80x9cSxe2x80x9d-shaped, with a curvature reversal or inflection at the middle. The center of this xe2x80x9cSxe2x80x9d shape crosses the shared axis of the two helices. The pitch of the helix may be increased over approximately the last full turn approaching the central crossover region from either end, to allow space for gripping the spring wire at its center with a clamp and for attaching the clamp to external attachments. Motion of the center attachment relative to the end attachments is generally parallel to the common axis, with spring restoring force also being exerted along this axis, which is designated the axis of spring action. Attachment at the center of the in-line embodiment may be made with a cylindrical clamp and soft clamping materials, similarly to attachment to the side-by-side embodiment, except that the clamp is likely to require an xe2x80x9cSxe2x80x9d-shaped inner curvature to fit the sharply bending wire. Attachment means for the ends of the in-line embodiment may be similar to those for the side-by-side embodiment.
The in-line and side-by-side embodiments of the invention share the property that when the moving attachment travels along the axis of spring action, as defined separately for each of the two embodiments, then the responsive spring force acts substantially parallel to the axis of spring action, while the center of the spring wire has very little tendency to twist about any axis or rotation. The symmetry balance giving rise to this low-twist property arises, in part, from the non-mirroring symmetry of the side-by-side spring with its reversal of axial progression along the wire, and from the mirroring symmetry of the in-line spring with no reversal of axial progression along the wire.